The present invention relates generally to an apparatus for a tractor having two tracks. More specifically, the apparatus is an improved configuration for a tractor having two track assemblies and a wheel assembly in front of the track assemblies, which wheel assembly is automatically controlled to adjust an amount of weight on the wheel assembly, for example, to limit or eliminate slip and thereby an amount of wear on tread bars of the track assemblies. Such apparatus thereby solves multiple problems encountered with current two-track tractors using two rubber belt tracks.
Both a two-track tractor configuration and a half-track tractor configuration are well known. The earliest large farm tractors often were half-track configurations having a combination of steel tracks and wheels. For traction, tracks were chosen because they provided better performance characteristics than wheels. For mobility in soft terrain, tracks were better than wheels.
The original main purpose of the wheel mechanism in front was to steer the tractor. Tracks had good characteristics for fieldwork. In order to steer, however, a significant percentage of the total weight had to be carried on the front wheel mechanism. This caused the tractor to be extra heavy. Partial sinking of the front wheel mechanism compromised the conventional tractor's mobility in soft terrain. Later, a steering system was developed that relied on creating a track speed difference between the two tracks. This speed difference system worked well for steering and allowed for the front wheel mechanism to be removed. Removal of the front wheel mechanism allowed for all of the tractor's weight to be used for traction purposes and for mobility improvements in soft terrain.
Steel tracks with sharp grouser tips have a disadvantage of not being able to travel on paved roads because they tend to damage the road's surface and because of slow travel speeds. With the advent of paved roads and farmers' fields becoming farther apart, the use of steel tracks came into disfavor. The preferred tractor configuration became the wheel tractor using only tires. For tractors that are used primarily for pulling heavy loads, it is preferred that all wheels be powered in order that all the weight can contribute to drawbar forces. The two main steering systems that are currently used for wheel tractors are Ackerman steer for small tractors and articulated steering for large tractors.
Around 1985, a two-track crawler tractor became available that used rubber belt tracks, as distinguished from steel tracks. This crawler tractor with rubber belt tracks could now travel on the road without damage to the road and also travel at higher speeds both in the field and on the road. An advanced steering system, known in the art as differential steer, was further developed and used on this two-track crawler tractor.
Currently, there are two different rubber belted Ag tractor configurations available in the market. One configuration has two tracks and the other configuration has four tracks.
The dynamics of the two-track configuration include the following: the standard two-track configuration has a steady state harsh ride during road travel because the amount of weight on the track assembly's front spindle is excessive during road travel and because the rear of the tractor has no suspension. This excessive amount of weight up front also causes rapid wear-out of the rubber belts.
The process causing rapid wear-out involves the motion of the rubber belt's portion that is wrapped around and is traversing the front spindle's wheel (see FIG. 1). This wheel is located inside the belt, at the front of the belt, and is called the front idler. This portion of the tread bar's tips, when traveling tangentially around the front idler, travels faster relative to a baseline speed than when such portion travels on the straight portions of the belt. The baseline speed is here defined as the speed of the belt's inextensible reinforcement package. This faster travel about the idler happens because the tread bar tips, when traveling in an arc around the front idler, are at a larger radius than is the radius of the belt's reinforcement package, measured from the rotational center of the front idler.
This speed ratio of the tread bar's tips to the reinforcement package is the direct ratio of these two radii. That is, referencing the belt's inextensible reinforcement package as being the speed of the vehicle, the portion of tread bar tips traveling tangentially around the front idler are moving faster than the speed of the vehicle. For that matter, it is generally known by those skilled in the art that when tire tread bars or belt tread bars travel at a speed different than the vehicle's speed, the phenomena “slip” occurs. Slip causes wear on the tread bar tips and the rate of wear is proportional to the contact force between the tip and the ground.
The process causing rapid wear-out of tread bars is specifically related to the events happening directly below and slightly ahead of the front idler's centerline, at a point where tread bar tips are starting to engage the ground, i.e., transitioning from an unloaded state to a loaded state.
As depicted in FIG. 1, when the standard two-track (only the right track 12 is shown in FIG. 1) tractor configuration is traveling on the road, the load on the front idler 14 is excessive, as indicated by the load (L) as a function of positioning along the length of the tracks with respect to the direction of travel. Clearly, the load at position “I” of the track, directly under the idler axle centerline 14A (which is coincident with the tipping fulcrum), is where the maximum load or weight is realized. This excessive load causes excessive compression in the tread bars below the front idler's centerline 14A. This compression, in turn, causes the initial contact point of the tread bars to happen several centimeters forward of the idler's centerline.
As described above, the tread bar's tips contact the ground with slip. Slip continues as long as the belt is in a condition of being wrapped around the front idler. During the several centimeters of slip, the contact force between the tread bar's tip and the ground increases from zero to the maximum value (at “I” as shown in FIG. 1). The result is a rapid rate of wear on the tread bar's tip at this location (i.e., proximate the front idler 14) when the tread bar's tip is transitioning from an unloaded state to a loaded state.
The harsh ride referred to above results from this transitioning of the tread bar's tips from an unloaded condition to a condition of excessive load in combination with the large spacing of the tread bars, i.e., about 8″ or 200 mm. This is the fore aft spacing between belt's tread bars. This creates a ride effect similar to that created by rumble strips on a highway. This dynamic situation translates into the need to frequently replace expensive rubber track belts.
It is also known that tractors tend to transfer vertical load off their front support means and onto their back support means when pulling a drawbar load. Drawbar pull generates a pitch moment that is approximately equal to the amount of drawbar pull multiplied by drawbar height. The vertical load transfer is the reaction to the pitch moment. For the standard two-track configuration, drawbar pull transfers vertical load off the front of the track assemblies and onto the rear of the track assemblies.
For the rubber belt tracks to have their best traction efficiency and best floatation characteristics, it is necessary to have the tractor's weight equally distributed under the track assemblies; front to back. The standard two-track configuration typically uses counterweights at the front of the tractor's center structure intending to accomplish an equally distributed weight profile under the track assemblies, front to back, during fieldwork that normally encounters large amounts of drawbar pull. This situation is referred to as being properly balanced for fieldwork.
When the tractor's balance is biased for a large amount of drawbar pull in the field, then it is improperly balanced when traveling on the road where small amounts of drawbar pull are the norm, as shown in FIG. 1. Exceptions to the norm include towing heavy wagons up steep grades. The normal situation, when traveling on the road, is each track assembly's front idler carries a large percent of the vehicle's weight.
During certain operations in the field, the amount of drawbar pull tends to vary as the tractor traverses the field. This, in turn, causes the weight distribution under the tracks to vary in proportion to varying drawbar pull as the two-track tractor configuration traverses the field. Non-uniform weight distribution under the tracks increases fuel consumption, increases track slippage, increases track wear, and increases soil compaction.
For good traction characteristics in the field, it is desirable to use an aggressive tread bar pattern, such as a chevron pattern, on the belt and to space the tread bars about 200 mm (8 inches) inches apart. When traveling on the road, this large spacing of the tread bar, in combination with the heavy load on each track assembly's front idler, is the source of an annoying and continuous vibration for the operator to tolerate: like a continuous rumble strip.
Another situation occurring during field operations is the standard two-track tractor configuration pitches harshly under certain conditions such as traveling at right angles over a ridge. The pitch motion may cause the operator to experience discomfort, injury, or lose control of the tractor.
The standard two-track tractor configuration is not suspended at the rear axle. This further contributes to operator discomfort and high stresses within the tractor's structure.
During a steering maneuver, the standard two-track tractor configuration often encounters a braking action on the track assembly that is on the inside of the turn radius. This is because an inside track braking force is often needed to overcome lateral traction forces. Other situations exist when both track assemblies are experiencing braking actions and when the brake forces need to be uneven from side to side for steering purposes, e.g., when a tractor is pushed by a heavy wagon when going down a steep grade and maximum brake capacity is needed in order to maintain the operator's maximum allowed speed.
If a steering correction is needed and, because the inside track of the steering correction's turn radius is at its braking limit, the track on the outside of the turn radius must then give up some of its brake force in order to make a steering correction. The net result is the velocity of the tractor's centerline and of the wagon increases. The brakes are then unable to decelerate the tractor and wagon back to the operator's maximum allowed speed.
Another possible result caused by the wagon's pushing the tractor is a resulting forward pitch moment that further increases the vertical load on the front of the track assemblies and lessens the load on the rear. The resulting forward pitch amplitude increases the height of the drawbar and in turn pitches the wagon's tongue backwards, imposing an upward jackknife between the tractor's drawbar and the wagon's tongue more likely.